1. Field of the Invention
The present invention relates to an image recording apparatus such as an electrophotographic printer which utilizes a developer containing toner and magnetic carrier particles.
2. Description of the Related Art
In an electrophotographic printer which utilizes a developer containing toner and magnetic carrier particles, the toner concentration in a developing unit lowers due to consumption thereof as the printing operation proceeds. To make up for the toner consumption, therefore, a toner sensor is provided for detecting the toner concentration within the developing unit, so that the toner is automatically supplied into the developing unit according to the output voltage of the toner sensor.
Typically, the toner sensor detects the toner concentration on the basis of magnetic permeability, so that the output voltage of the sensor increases with a decreasing toner concentration. At the start of a printing operation, since the developer is not agitated well nor charged sufficiently, the output voltage of the toner sensor tends to become high. More specifically, as shown in FIG. 9 of the accompanying drawings, the output voltage of the toner sensor rises rapidly to a peak level P immediately after starting agitation of the developer and subsequently drops gradually to a steady level at a constant gradient even if the toner concentration is kept constant.
In a prior art printer, therefore, light exposure of the photosensitive drum is started only upon lapse, from the start of developer agitation, of a wait time t.sub.1 in which the output voltage of the toner sensor reaches the peak level P. Subsequently, the so-called floating control of toner supply follows wherein the output voltage of the toner sensor is sampled periodically for comparison with a varying standard voltage (similar to the voltage curve shown in FIG. 9) and wherein an amount of toner is supplemented into the developing unit if the output voltage of the toner sensor is higher than the standard voltage by no less than a predetermined amount.
With the prior art printer described above, since the light exposure of the photosensitive drum is started only upon lapse of the wait time t.sub.1 of about 7-8 seconds after starting the rotation of the drum, the time required for printing the first page is corresponding prolonged. Further, since the photosensitive drum, the toner agitation rollers and the magnet roll are rotated even during the wait time, the service life of the drum and/or the developer is inevitably shortened.